Surgical implant

ABSTRACT

A surgical implant comprises a core region and a porous surface region extending over at least a part of said core region. The porous surface region has a predetermined pore volume fraction. A method of manufacturing a surgical implant in accordance with the invention comprises the steps of: (i) loading metallic powder having a predetermined particle size distribution around a pre-formed core in a sealable capsule; (ii) reducing pressure within said capsule to a predetermined pressure below atmospheric pressure; (iii) pressurising said capsule with a process gas to a predetermined pressure higher than the predetermined pressure of step (ii); (iv) sealing said capsule; (v) heating said pressurised sealed capsule at an elevated temperature and an elevated pressure for a predetermined time to produce an implant precursor; (vi) cooling said sealed capsule, and; (vii) heating said implant precursor for a predetermined time at an elevated temperature and a predetermined pressure below atmospheric pressure, whereby to generate porosity in said implant precursor.

[0001] The present invention relates to a surgical implant and tomethods of making the same.

[0002] There are many different types of surgical implants and devices,including orthopaedic implants (hip, shoulder, knee, ankle, elbow),cranio-facial implants and spinal implants (such as a spinal cage orspinal disc). Such implants are commonly made from biocompatible metalssuch as titanium, titanium-based alloys, stainless steel orcobalt-chromium alloys, depending on strength and weight considerations.Another consideration, particularly for implants which replace bone, isthe elastic modulus of the implant. The closer the modulus of elasticityis to natural bone, the better the stress redistribution to adjacentbone. Better stress distribution results in prolonged useful life of theimplant.

[0003] To increase bonding of surrounding tissues with an implant, it isknown to apply a coating to the surface of the implant, by for exampleplasma spraying, which roughens the surface of the implant. The coatingmay be of a different composition to the implant to improve wearresistance and/or to provide enhanced biocompatibility (eg. TiN isextremely inert and can be applied as a coating to aluminium- orvanadium-containing implants to prevent leaching of those metals fromthe implant.

[0004] It is an object of the present invention to provide an improvedsurgical implant, which preferably has improved bone compatibilityand/or wear resistance and/or useful life and/or is stronger than knownimplants.

[0005] According to the present invention, there is provided a surgicalimplant comprising:

[0006] (i) a core region, and

[0007] (ii) a porous surface region extending over at least a part ofsaid core region,

[0008] wherein the porous surface region has a predetermined pore volumefraction.

[0009] It will be understood that as used herein, “pore volume fraction”of a porous region is a ratio of pore volume in that region to the totalvolume of that region, expressed as a percentage.

[0010] The implant may be any shape as necessitated by its intendedapplication. For example the implant may be elongate (eg. cylindrical)or disc-shaped. The implant may have an irregular shape, and the poroussurface region may vary in thickness over the core region.

[0011] Preferably, the pore volume fraction within the porous surfaceregion is 20 to 50%. The predetermined pore volume fraction is chosenaccording to the nature of the implant. A higher pore volume fractionresults in a lighter implant with a lower modulus of elasticity.

[0012] Preferably, the pores are interconnected and substantiallyuniformly distributed within the porous surface region. Preferably, atleast some (and preferably the majority) of the pores have a size in therange of from 100 μm to about 750 μm, and more preferably from about 200μm to 500 μm.

[0013] Preferably, the porous surface region is at least about 1 mmthick, and is more preferably from about 2 mm to about 5 mm thick, mostpreferably 2 mm to 3 mm thick.

[0014] Distinct regions within the porous surface region may have adifferent pore size distribution and/or a different pore volume fractionsuch that there is a pore size and/or pore volume fraction gradientwithin the porous surface region. For example, there may be a porevolume fraction gradient along an axis of the implant and/orperpendicular to the axis of the implant.

[0015] The core region may be fully dense or porous. The degree ofporosity of the core region may be more than, less than or the same asthe surface region.

[0016] Preferably, the core region is relatively less porous than thesurface region.

[0017] Preferably, the core region has a density of from 0.7 to 1.0 oftheoretical density (i.e. from 0 to 30% porosity).

[0018] The core region and the porous surface region may be constructedfrom the same or different materials. Particularly suitable materialsinclude titanium (eg. commercial purity [ASTM B 338 GR 2] titanium),stainless steel, titanium-based alloys (eg. Ti—Al—V alloys and Ti—Al—Nballoys) and cobalt-chromium based alloys.

[0019] Particularly preferred materials are commercial purity titanium,Ti-6Al-4V, Ti-6Al-7Nb, Stellite 21 and stainless steel 316L.

[0020] Preferably, the porous surface region is diffusion bonded to thecore region. Thus, it will be understood that the interface between thecore region and the porous surface region does not introduce a weaknessinto the implant.

[0021] According to a second aspect of the invention, there is provideda method of manufacturing a surgical implant in accordance with saidfirst aspect comprising the steps of:

[0022] (i) loading metallic powder having a predetermined particle sizedistribution around a pre-formed core in a sealable capsule,

[0023] (ii) reducing pressure within said capsule to a predeterminedpressure below atmospheric pressure,

[0024] (iii) pressurising said capsule with a process gas to apredetermined pressure higher than the predetermined pressure of step(ii),

[0025] (iv) sealing said capsule,

[0026] (v) heating said pressurised sealed capsule at an elevatedtemperature and an elevated pressure for a predetermined time to producean implant precursor,

[0027] (vi) cooling said sealed capsule, and

[0028] (vii) heating said implant precursor for a predetermined time atan elevated temperature and a predetermined pressure below atmosphericpressure, whereby to generate porosity in said implant precursor.

[0029] Preferably, the powder used in step (i) has a particle size rangeof from about 50 μm to about 750 μm (i.e. substantially all theparticles are within the specified size range). The powder may beproduced, for example, by gas atomisation or mechanical attrition.

[0030] Conveniently, the core may be integral with the capsule, althoughthe core may be free standing. In either case, the core may be of thesame or different material to the powder and of the same or differentmaterial to the rest of the capsule.

[0031] Preferably, the predetermined pressure of step (ii) is 10⁻³ mbar(0.1 Pa) or less. Preferably, the predetermined pressure of step (vii)is 10⁻³ mbar (0.1 Pa) or less.

[0032] The process gas of step (iii) may be an unreactive gas (eg.argon), or a reactive gas (eg. nitrogen), or a mixture of reactive andunreactive gases, the unreactive gas serving as a carrier or diluent forthe reactive gas. When nitrogen is present in the process gas, thesurfaces of the eventually formed pores become nitrided therebyincreasing wear resistance and chemical inertness. Use of nitrogen isparticularly advantageous when the metallic powder contains titanium. Asan alternative, a separate nitriding step may be carried out after step(vii), preferably at a pressure of from about 800° C. to about 1000° C.and at a pressure of from about 10 to 100 MPa for 1 to 8 hours.

[0033] Preferably the pressure of step (iii) is from about 1 bar (1×10⁵Pa) to about 5 bar (5×10⁵ Pa) positive pressure.

[0034] Preferably, the elevated temperature of step (v) is from about850 to about 1100° C. Preferably, step (v) is carried out over a periodof from about 1 hour to about 4 hours.

[0035] In step (vi), the sealed capsule (and the implant precursortherein) is preferably cooled to room temperature. Step (vii) may beperformed on the implant precursor in the capsule. Alternatively, theimplant precursor can be separated from the capsule after step (vi)prior to step (vii). The capsule is conveniently removed by, forexample, machining it off on a lathe. Preferably, the elevatedtemperature of step (vii) is from about 900 to about 1300° C.Preferably, step (vii) is carried out for no more than about 80 hours,preferably no more than about 60 hours, and even more preferably no morethan about 48 hours. Preferably, step (vii) is carried out for at least6 hours and more preferably for at least 12 hours.

[0036] If step (vii) is performed with the precursor implant in thecapsule, the capsule is separated from the implant after step (vii), byfor example machining.

[0037] It will be understood that the implant formed by the above methodwill have the shape of the interior surfaces of the capsule so thatadditional machining steps may be required to obtain the final requiredshape for the implant. However, an implant having a shape substantiallyas required can be produced by using a capsule having an appropriatelyshaped interior surface. Examples of methods by which an appropriatelyshaped capsule can be made include electro-forming and direct laserfabrication.

[0038] In a slight modification of the above method, the metallic powderis partly consolidated prior to encapsulation. This may be achieved by,for example, selective laser sintering a mixture of the metallic powderand a polymeric binder. Different powder fractions may be used to formdifferent regions of the implant, and so this modification isparticularly useful for making implants having a gradient porositywithin the porous region.

[0039] According to a third aspect of the present invention, there isprovided a further method of manufacturing a surgical implant of saidfirst aspect, comprising the steps of:

[0040] (i) selectively sintering successive layers of metallic particleswhereby to form an implant precursor of required shape, and

[0041] (ii) heating said implant precursor whereby to form said implant.

[0042] Preferably, step (ii) is effected under reduced pressure, forexample in a vacuum oven.

[0043] Step (i) may be effected by sintering the particles around acore. Preferably, said core is from 0.7 to 1.0 of theoretical density(i.e. has a porosity of from 0 to 30%). However, the core may have thesame, higher or lower density, than the resultant porous surface region.

[0044] Preferably, substantially all said metallic particles have a sizeof 750 μm or less, more preferably 500 μm or less and most preferably150 μm or less. Preferably, said particles have a size of at least 50μm. Preferably, said metallic particles are in admixture with a bindersuch as a light and/or heat sensitive polymeric binder. More preferably,step (i) is effected by scanning a laser (typically of power 20 to 40 W)over said mixture of metallic powder and binder.

[0045] Preferably, step.(ii) is effected from about 1000° C. to about1300° C. It will be understood that in step (ii), any binder present isburnt off.

[0046] It will be understood that step (i) can be effected to giveregions of macroporosity. For example, the laser sintered region can beprovided with recesses extending inwardly from the surface of theimplant, and/or chambers within the implant. In addition, said chambersand/or recesses may be interconnected to provide channels or passages inthe implant. It will also be understood that such macroporosity can beintroduced into the implant produced by the modified method of thesecond aspect which uses selective laser sintering.

[0047] Embodiments of the invention will now be described by way ofexample only, with reference to the accompanying drawings in which:

[0048]FIG. 1 is a photograph showing how macroporosity can be introducedby the method according to the third aspect of the invention, and

[0049]FIGS. 2a is a schematic sectional representation of a replacementlumbar spinal disc in accordance with the present invention,

[0050]FIG. 2b is a plan view of the disc of FIG. 2a,

[0051]FIG. 2c is a detail view of FIG. 2a,

[0052]FIG. 3a is a schematic sectional representation of a spinal cagein accordance with the present invention,

[0053]FIG. 3b is a plan view of the cage of FIG. 3a, and

[0054]FIG. 3c is a detail view of the cage of FIG. 3a.

EXAMPLE 1

[0055] A 100 mm long cylindrical container of commercial purity titanium(ASTM B 338 GR 2) having an internal diameter of about 22 mm and anexternal diameter of 25.4 mm was filled with a gas atomised powderconsisting of particles of Ti-6Al-4V alloy (Crucible Research, USA)having a predetermined particle size distribution. Where necessary, thepowder was passed through sieves of appropriate mesh to obtain aparticular particle size fraction. The container was filled withoutvibration, i.e. to “tap density” only. Air was evacuated from thecontainer (pressure<0.1 Pa) and back-filled with argon (10⁵ Pa positivepressure) for 2 minutes. After sealing, the container was HIPped for 4hours at 950° C. at an external pressure of 100 MPa. Subsequently, thecontainer was allowed to cool to room temperature and atmosphericpressure, Ar remaining under pressure in the alloy matrix. The implantprecursor was machined out of the container and heat treated at 1050° C.for 48 hours under vacuum (<0.1 Pa).

[0056] The degree of porosity in the resultant implant is given fordifferent powder fractions in Table 1 below. TABLE 1 Mean pore PowderPorosity size Size Distribution of Pores (μm) Ex. (μm) (%)(μm) >350 >250 >150 >100 >50 1a <125   32 ± 2.3 32 — — — 4 47 1b <15032.1 ± 1.0 43 — — — 6 51 1c <500 31.3 ± 2.0 49 — — 2 14 59 1d 125-150  34 ± 1.5 61 — — 6 22 68 1e 150-180 34.6 ± 2.0 74 — 1 14 33 72 1f180-250 27.8 ± 2.7 72 — 1 13 34 73 1g 150-250 34.2 ± 3.0 72 0.2 2 14 3172 1h 150-500 34.2 ± 1.7 102 1.3 3 20 41 76 1i 250-425 28.2 ± 1.9 1191.5 8 28 48 80 10 250-500 33.2 ± 2.5 118 2.2 9 28 46 77 1j 425-500 20.1± 2.2 116 1.7 9 27 45 77 1k 500-750 20.7 ± 3.6 111 2.6 9 24 41 69

EXAMPLE 2

[0057] Example 1 was repeated using powders having a size distributionof 180-250 μm and 150-500 μm. HIPping was carried out at 1000° C. andsubsequent heat treatment (after removal from the container) was at 1050° C. for 48 hours. The results are shown in Table 2. TABLE 2 Mean porePowder Porosity size Size Distribution of Pores (μm) Ex. (μm) (%)(μm) >350 >250 >150 >100 >50 2a 180-250 40.7 ± 3.3 96 0.3 6 27 47 80 2b150-500 38.8 ± 2.1 117 1.3 7 27 48 78

EXAMPLE 3

[0058] Example 1 was repeated using a powder having a size distributionof 250-500 μm. HIPping was carried out at 1000° C. and subsequent heattreatment (after removal from the container) was at 1050° C. from 12 upto 48 hours. The results are shown in Table 3. TABLE 3 Mean poreTreatment Porosity size Size Distribution of Pores (μm) Ex. Time (hr)(%) (μm) >350 >250 >150 >100 >50 3a 12 33.2 ± 1.5 117 2.9 9 26 44 77 3b24 33.8 ± 2.1 120 3.0 10 28 46 76 3c 36 36.1 ± 3.0 123 4.2 12 28 44 763d 48 33.2 ± 2.5 118 2.2 9 28 46 77

EXAMPLE 4

[0059] Example 1 was repeated using a powder having a size distributionof <150 μm. Subsequent heat treatment after removal of the container wasat 1050° C. The results are shown in Table 4. TABLE 4 Mean poreTreatment Porosity size Size Distribution of Pores (μm) Ex. Time (hr)(%) (μm) >350 >250 >150 >100 >50 4a 12  4.1 ± 0.7 15 — — — — — 4b 24  16 ± 3.0 24 — — — — 7 4c 36 25.6 ± 5.8 27 — — — 2 12 4d 48 28.3 ± 3.631 — — — 3 17

EXAMPLE 5

[0060] Example 1 was repeated using a powder having a size distributionof 250-425 μm. Subsequent heat treatment after removal of the containerwas for 48 hours at different temperatures. The results are shown inTable 5. TABLE 5 Mean Treatment pore Temperature Porosity Size SizeDistribution of Pores (μm) Ex. (° C.) (%) (μm) >350 >250 >150 >100 >505a 1050° C. 29 ± 1 117 1.0 6 27 47 82 5b 1100° C. 26 ± 1 108 0.5 5 23 4380 5c 1150° C. 26 ± 3 102 0.2 4 20 41 73

EXAMPLE 6

[0061] Example 1 was repeated with various powders. After the heattreatment (1100° C. for 48 hours) the implants were pressure nitrided,in pure nitrogen, at 950° C. and 50 MPa for 6 hours to coat the internalsurface of the pores with a thin layer of titanium nitride. The resultsare shown in Table 6. TABLE 6 Mean pore Powder Porosity size SizeDistribution of Pores (μm) Ex. (μm) (%) (μm) >350 >250 >150 >100 >50 6a<125 44.7 ± 3.7 69 0 0 3 16 62 6b 125-150 44.8 ± 4.7 93 0 1 14 38 77 6c150-180 40.8 ± 2.7 107 1.0 4 21 43 80 6d 180-250 34.7 ± 3.1 114 0.4 3 2341 70 6e 250-425 31.2 ± 2.6 146 5 15 37 59 89 6f 425-500   21 ± 3.1 1433 13 39 61 86

EXAMPLE 7

[0062] A 12.7 mm solid cylindrical core of Ti-6Al-4V was centrallyplaced in the titanium container described above for Example 1. Thespace between the core and the inner cylinder wall was filled withTi-6Al-4V powder (particle size 500-710 μm). The container was evacuatedand backfilled with Ar to 10⁵ Pa positive pressure, sealed and HIPped at950° C. for 4 hours at 100 MPa. Subsequent heat treatment after removalfrom the container was at 1050° C. for 48 hours. This resulted in asample having a porous coating of 29±5% porosity with 43% of pores >50μm, 16%>100 μm and 6%>150 μm.

EXAMPLE 8

[0063] Example 1 was repeated using powder having a particle size <125μm. After HIPping the part is machined to remove the container and toreduce the diameter of the part to ˜17 mm. Unlike Example 1, no heattreatment was carried out at this stage. The resultant cylinder was thenused as the core in the same way as Example 7, the container having anexternal diameter of 38.1 mm and an internal diameter of approx 35 mm.Example 7 was then repeated, the core being surrounded in this case with250-425 μm size powder. The resultant implant had a porous core with amean pore size of 26 μm (28.9±2.10% porosity) and a porous outer layerwith a mean pore size of 113 μm (24.9±1.9% porosity).

[0064] Example 8 demonstrates the ability of the method to produceimplants having predetermined regions of differing porosity in acontrollable manner. In addition, it will be understood that theprocedure can be extended to produce an implant having more than 2different layers of differing porosity, i.e. graded porous structurescan be manufactured.

EXAMPLE 9

[0065] Example 1 was repeated using powder having a predetermined sizedistribution. After the heat treatment (1100° C. for 48 hours), sampleswere machined into tensile test pieces such that the properties of theporous material could be assessed. The results are shown in Table 7.TABLE 7 0.2% 0.3% Powder Porosity UTS E Elongation PS PS Ex. (μm) (%)(MPa) (GPa) (%) (MPa) (MPa) 7a <500 24.6 386 58 1 346 367 7b 125-15025.7 437 60 1.1 383 398 7c 150-250 26.2 435 64 0.8 384 399 7d 250-50027.5 493 74 1.8 420 439

EXAMPLE 10

[0066] In order to demonstrate that in the Examples where a pre-formedcore is bonded to a porous surface region, there is a strong bond at theinterface between the core and surface regions, tensile strength testswhere carried out. In these tests, the powder (Ti-6Al-4V) wassimultaneously consolidated and diffusion bonded to a solid Ti-6Al-4Vbar such that the interface was at the midpoint of the gauge length ofthe sample. Two powder fractions were assessed, 125-150 μm (test samplea) and 150-250 μm (test sample b). In each case, the sample containerwas backfilled with Ar to 10⁵ Pa positive pressure. The initial HIPstage was carried out at 950° C. for 4 hours at 100 MPa. The subsequentheat treatment stage was carried out at 1100° C. for 48 hours. Theresultant porosity was about 40% (test sample a) and 35% (test sampleb).

[0067] In both cases the tensile UTS was about 400 MPa and failureoccurred remote from the interface within the bulk of the porousmaterial. Routine metallographic investigations confirmed that theinterface remained unaffected and that the interface was free ofcontamination (i.e. oxides) and indistinguishable from the adjacentmatrix.

EXAMPLE 11

[0068] Referring to FIG. 1, the parts shown were fabricated by selectivelaser sintering. The laser is controlled by a computer which is loadedwith a CAD file containing the configuration of the part. The part wasfabricated on a powder bed heated to 160° C. (typical temperature100-200° C.). Powder of particle size <150 μm and 15% by volume binder(typically 5 to 25% by volume) was heated to 60° C. (typically 50-100°C. and fed onto the powder bed with a build height of 200 μm per layer(typically 100 to 500 μm). Selective sintering was effected using a 25 Wlaser. After the part had been built up, debinding was effected at 400°C. for 30 minutes (typically 200-500° C. for 30 to 120 minutes) afterwhich sintering was effected at 1100° C. (typically 1000-1350° C.).

[0069] As can be seen from FIG. 1, the part 2 has a generally porouscylindrical region (porosity of about 20-25%), with a regular array ofround recesses 6 around its outer periphery (“macroporosity”). Therecesses 6 are 0.75 mm in circumference and extend inwardly by 3 mm.Part 12 is similar to part 2 except that the recesses 6 are 2 mm indiameter.

[0070] Part 22 is also similar to part 2, except that the recesses 6 are1 mm in diameter and 5 mm deep. In addition, a circular array ofchannels 8 is provided. Each channel is 1.5 mm in diameter and extendsfrom the top surface of the part to its base.

[0071] Referring to FIGS. 2a to 2 c, a replacement spinal disc is shownschematically. The disc comprises a per se known biomedicalsemicrystalline polymeric disc 30 (for example polyurethane orpolyethylene) sandwiched between a pair of disc-shaped metallic endplates 32 in accordance with the present invention. The polymeric disc30 is secured to the end plates 32 by adhesive, although it will beunderstood that the elements can be fused together or the end plates 32infiltrated with the polymeric disc 30.

[0072] The discs 32 themselves are generally kidney shaped (FIG. 2b) andcomprise a substantially non-porous core region 32 a adjacent thepolymeric disc 30 and a porous outer region (32 b), remote from thepolymeric disc (30) (FIG. 2c). The porous outer region facilitatestissue in-growth.

[0073] Referring to FIGS. 3a to 3 c, a spinal cage 40 is generally inthe shape of a hollow tuber with a rectangular cross section. A core 42of substantially fully dense metal is surrounded by a porous outer metalregion 44 (facilitating in growth of tissue). A passage 46 extendsthrough the centre of the cage 40.

[0074] It will be readily apparent from the foregoing that by carefulselection of powder fraction and process parameters, a desired porosityand pore size distribution of the porous surface region can be obtained,according to the particular end application intended for the implant.Although the above Examples are cylindrical rods, it will be understoodthat they can be readily machined to an appropriate shape depending onthe proposed use. Equally, it will be understood that the container usedto prepare the implant precursor need not be cylindrical and can be ofany desired shape.

1. A surgical implant comprising: (i) A core region, and (ii) a poroussurface region extending over at least a part of said core region,wherein the porous surface region has a predetermined pore volumefraction.
 2. An implant as claimed in claim 1, wherein the pore volumefraction within the porous surface region is 20 to 50%.
 3. An implant asclaimed in claim 1, wherein the pores are interconnected andsubstantially uniformly distributed within the porous surface region. 4.An implant as claimed in claim 1, wherein at least some of the poreshave a size in the range of from 100 μm to about 750 μm.
 5. An implantas claimed in claim 1, wherein the porous surface region is at leastabout 1 mm thick and preferably from about 2 mm to about 5 mm thick. 6.An implant as claimed in any claim 1, wherein distinct regions withinthe porous surface region have a different pore size distribution and/ora different pore volume fraction such that there is a pore size and/orpore volume fraction gradient within the porous surface region.
 7. Animplant as claimed in claim 1, wherein the core region has a density offrom 0.7 to 1.0 of theoretical density.
 8. An implant as claimed inclaim 1, wherein the core region and/or the porous surface region aremade from titanium, commercial purity [ASTM B 338 GR 2] titanium,stainless steel, titanium-based alloys, Ti—Al—V alloys, Ti—Al—Nb alloysor cobalt-chromium based alloys.
 9. An implant as claimed in claim 8,wherein the core region and/or the porous surface region are made fromTi-6Al-4V, Ti-6Al-7Nb, Stellite 21 or stainless steel 316L.
 10. A methodof manufacturing a surgical implant in accordance with claims 1comprising the steps of: (i) loading metallic powder having apredetermined particle size distribution around a pre-formed core in asealable capsule, (ii) reducing pressure within said capsule to apredetermined pressure below atmospheric pressure, (iii) pressurisingsaid capsule with a process gas to a predetermined pressure higher thanthe predetermined pressure of step (ii), (iv) sealing said capsule, (v)heating said pressurised sealed capsule at an elevated temperature andan elevated pressure for a predetermined time to produce an implantprecursor, (vi) cooling said sealed capsule, and (vii) heating saidimplant precursor for a predetermined time at an elevated temperatureand a predetermined pressure below atmospheric pressure, whereby togenerate porosity in said implant precursor.
 11. A method as claimed inclaim 10, wherein the powder used in step (i) has a particle size rangeof from about 50 μm to about 750 μm.
 12. A method as claimed in claim10, wherein the predetermined pressure of step (ii) is 10⁻³ mbar (0.1Pa) or less.
 13. A method as claimed in claim 10, wherein thepredetermined pressure of step (vii) is 10⁻³ mbar (0.1 Pa) or less. 14.A method as claimed in claim 10, wherein the process gas of step (iii)is an unreactive gas, or a reactive gas, or a mixture of reactive andunreactive gases, the unreactive gas serving as a carrier or diluent forthe reactive gas.
 15. A method as claimed in claim 14, wherein nitrogenis used as the reactive gas.
 16. A method as claimed in claim 10,wherein a nitriding step is carried out after step (vii).
 17. A methodas claimed in claim 10, wherein the pressure of step (iii) is from about1×10⁵ Pa to about 5×10⁵ Pa positive pressure.
 18. A method as claimed inclaim 10, wherein the elevated temperature of step (v) is from about 850to about 1100° C.
 19. A method as claimed in claim 10, wherein theelevated temperature of step (vii) is from about 900 to about 1300° C.20. A method as claimed in claim 10, wherein the implant precursor isseparated from the capsule after step (vi) prior to step (vii).
 21. Amethod as claimed in claim 10, wherein the capsule is separated from theprecursor implant after step (vii) by machining.
 22. A method as claimedin claim 10, wherein the metallic powder is partly consolidated prior toencapsulation, preferably by selective laser sintering a mixture of themetallic powder and a polymeric binder.
 23. A method of manufacturing asurgical implant in accordance with claim, comprising the steps of: (i)selectively sintering successive layers of metallic particles whereby toform an implant precursor of required shape, and (ii) heating saidimplant precursor whereby to form said implant.
 24. A method as claimedin claim 23, wherein step (ii) is effected under reduced pressure.
 25. Amethod as claimed in claim 23, wherein step (i) is effected by sinteringthe particles around a core.
 26. A method as claimed in claim 23,wherein substantially all said metallic particles have a size of 750 μmor less.
 27. A method as claimed in claim 23, wherein said particles arein admixture with a light and/or heat sensitive polymeric binder.
 28. Amethod as claimed in claim 27, wherein step (i) is effected by scanninga laser over said mixture of metallic powder and binder.
 29. A method asclaimed in claim 23, wherein step (ii) is effected at a temperature offrom about 1000° C. to about 1300° C.
 30. A method as claimed in claim23, wherein step (i) is effected to give regions of macroporosity.